The present invention relates to a semiconductor memory device, and more particularly to a semiconductor memory device having an improved refresh characteristic, and a method for operating the same.
FIG. 1 illustrates a memory cell structure of a conventional semiconductor memory device.
The conventional semiconductor memory device includes a plurality of memory cells 101 to 108 each storing one data. Each of the memory cells 101 to 108 consists of one capacitor for storing the data, and one transistor for opening/closing the memory cell under control of a word line from WL<0> to WL<n>. That is, each of the memory cells 101 to 108 has a one-transistor & one-capacitor (1T-1C) structure.
In order to write/read data in a specific memory cell, the memory cell must be selected from the memory cells 101 to 108, which is performed by a row address of a word line from WL<0> to WL<n> and a column address of a pair of bit lines from BL0 to BL<k> and BLB0 to BLB<k> respectively. For example, in order to access the memory cell 101, a word line WL<0> is selected and enabled by the row address, and a pair of bit lines BL0 and BLB0 are selected by the column address. Then, data amplified by a bit line sense amplifier is input or output.
When data are stored in the capacitors of the memory cells, logic high data or logic low data are stored therein. However, the stored data are gradually lost by a leakage current as time elapses.
Accordingly, the semiconductor memory device periodically performs a refresh operation for updating the stored data in order to prevent the loss of data. That is, the semiconductor memory device inevitably performs the refresh operation because of loss of data caused by characteristics of the memory cells. If the semiconductor memory device could less frequently perform the refresh operation by increasing a data retention time of the memory cell, the performance of the semiconductor memory device would naturally be improved.
Therefore, there is a need for a technology that can increase the data retention time of the memory cells.